An implantable cardioverter defibrillator (ICD) is a small battery-powered electrical impulse generator that is typically implanted in persons who are at risk of sudden cardiac death due to ventricular fibrillation. ICDs are generally programmed to detect cardiac arrhythmia and correct it by delivering a treatment of electricity to the appropriate location in the person's heart. ICDs monitor the rate and rhythm of the heart and deliver such electrical therapies when the electrical properties of the heart activity present some pre-defined variance from the norm.
FIG. 3 is an illustration of a typical electrocardiograph (ECG) trace 300 over a single cardiac cycle 35. The ECG trace 300 presents several negative and positive deflections that correspond to the different electrical sequences that a heart goes through during a typical heartbeat, such as cardiac cycle 35. During normal atrial depolarization, the primary electrical impulse is directed from the sino-atrial (SA) node, i.e., the heart's pacemaker, toward the atrial-ventricular (AV) node. It will then spread from the right atrium to the left atrium. The path of this primary electrical impulse results in a P wave deflection 301 in the ECG trace 300.
After filling with blood resulting from the atrial depolarization, the ventricles also depolarize to pump the blood into the aorta for distribution to the body and the pulmonary arteries for distribution to the lungs. This ventricular depolarization ideally results in a quick succession of wave deflections in the ECG trace 300: a Q wave deflection 302, an R wave deflection 303, and an S wave deflection 304. The collection of the Q wave deflection 302, R wave deflection 303, and S wave deflection 304, representing the ventricular depolarization in ECG trace 300, is referred to as the QRS complex 305. The illustrated QRS complex 305 represents an idealized deflection formation for a typical heartbeat, such as cardiac cycle 35. However, a normal heartbeat may not always present with all three waves of the QRS complex 305. Generally, any combination of presenting Q, R, or S waves will be referred to as the QRS complex 305.
Because the ventricles contain more muscle mass than the atria, the QRS complex 305 is much larger than the P wave deflection 301. The shape of the QRS complex 305 will typically change when there is an abnormal conduction of the electrical impulses within the ventricles. However, the shape of the QRS complex 305 may also change depending on which recording electrodes of the ECG detect the electrical impulses.
After pumping the blood from the ventricles through depolarization, the ventricles repolarize during which time the atria relax and refill with blood for the next heartbeat. The repolarization of the ventricles presents as a T wave deflection 306 in the ECG trace 300. The collection of the P wave deflection 301, the QRS complex 305, and the T wave deflection 306 represents the typical heartbeat in the ECG trace 300. A fourth section, which is not always reflected or measured in an ECG trace, such as ECG trace 300, is a U wave deflection 307. The U wave deflection 307 is thought to represent the repolarization of the papillary muscles or His/Purkinje fibers, which are part of the system that coordinates the depolarization of the ventricles.
The illustration of the ECG trace 300 represents an idealized shape of an ECG trace of a normal heartbeat. In practice, ECG traces may present quite differently from the idealized shape of the ECG trace 300. These different shapes may be due to many different factors which include not only heart abnormalities, but also include the mere position of the patient being measured (e.g., prone vs. supine) or a physiologic problem caused by a drug interaction or activity of the patient. ICDs generally include filters that filter out the P, Q, S, and T waves. Thus, the ICD expects to only detect the R wave deflection. However, if one of these external factors causes the T wave deflection amplitude to increase over the filter limits, the ICD will sense the T wave deflection (i.e., oversensing) which the ICD expects to be indicative of an R wave deflection. The ICD is programmed to interpret this particular quick succession of R wave deflections as ventricular tachycardia, which if accurately detected, could be a precursor condition to ventricular fibrillation. Therefore, the ICD will attempt to treat the condition to prevent the heart from going into ventricular fibrillation. In the situation where the ICD over senses the T wave deflection when ventricular tachycardia is not present, an inappropriate electrical therapy may be delivered to the patient. The delivery of electrical therapy to the patient when the patient is not in cardiac distress can be quite uncomfortable. Thus, it is desirable to reduce or eliminate false positives.
ICD manufacturers have attempted to minimize the oversensing problem by adding automatic sensing control (ASC). One method employed in an ASC is bigeminal avoidance. Bigeminy is a slightly abnormal heart arrhythmia that presents as a normal sinus heartbeat with a premature ventricular beat. Various algorithms analyze beat intervals to predict that a bigeminal beat pattern is not, in fact, a tachycardial episode that merits treatment. These bigeminal avoidance techniques generally work when the bigeminy is experienced on every single beat. However, when the bigeminal beat is not experienced on every beat, these avoidance techniques do not consistently prevent improper treatment. Moreover, additional algorithms used in ASC systems include timing parameters, such that if a patient experiences a high rate for more than a predefined period of time, treatment will be administered, even if this detected high rate is due to detection of the bigeminal beat. Therefore, even the methods in ASC systems to avoid improper administration of treatment do not always prevent such improper treatment.